Various methods for recovering rubber from rubber-bearing plant materials are presently known in the art. Most of these currently known processes of rubber extraction from rubber-bearing plants are based on organic solvents and wet-milling with pebble mills.
The earliest industrial processes in this regard were based on solvent extraction from Guayule, a rubber bearing shrub, as described in U.S. Pat. No. 982,373. However, these processes were found to be impractically difficult and expensive.
Similarly, U.S. Pat. No. 1,695,676 describes the use of high pressure gas to break down the cell walls of Guayule. However, the substantial clogging of the extraction equipment with porous fibers, resulting in solvent blockage, has greatly decreased the significance of this process.
In contrast, U.S. Pat. No. 4,136,131 describes the isolation of rubber from rubber-bearing plants by reducing the Guayule shrub in size with an extruder under non-aqueous conditions (based on compressive and shear forces), forming a plastic mass, and flaking the plastic mass, followed by solvent extraction with acetone and cyclohexane. Higher rubber yield can be achieved by this process, however it is impractical in terms of cost-effectiveness. The process is also expensive and uses two expensive solvents which are harmful to the environment and explosive.
U.S. Pat. Nos. 4,526,959 and 4,684,715 each describe a process for rubber extraction from finely ground Guayule shrubs with organic solvents by percolation and monophase solvent systems, in particular the hexane/acetone monophase system. The fundamental problem in solvent extraction of rubber from plant materials is that rubber is a high molecular weight polymer unable to pass cell walls and membranous tissue in solution. This results in impractically slow extraction and very large solvent losses in pilot-plant operations.
Further, scientists have been trying to simulate mastication to extract the rubber in water and have demonstrated significant technologies for rubber recovery via wet-milling by pebble mills. Wet-milling by pebble mills was demonstrated with the Guayule shrub, Russian dandelion (Taraxacum kok-saghyz) and other species. Experiments using pebble mills in water, and further purification of rubber “worms” or whole plant, are described in U.S. Pat. Nos. 2,393,035; 2,399,156; 2,434,412; 2,459,369; 2,665,317 and 5,321,111.
Similarly, U.S. Pat. Nos. 2,393,035 and 2,399,156 disclose a process for recovering rubber from kok-saghyz and tau-saghyz by wet-milling with a pebble mill. Prior to wet-milling, the whole wet or dry roots are leached in hot water to remove carbohydrates, then the roots are extensively pebble milled in water to form a slurry of large tangled clots and crushed plant tissue. Upon dilution with water, this slurry is passed over a wet vibrating screen to separate the plant constituents from large tangled rubber clots. The root skins are then freed from the raw rubber by further pebble milling and waterlogging. The raw rubber obtained contains 10-15% of residual plant debris. Subsequently, this raw rubber is purified from the plant tissue debris by scrubbing in an NaOH solution followed by neutralization of sodium hydroxide with stearic acid.
The foregoing process was demonstrated on a pilot scale. (See RUSSIAN DANDELION (KOK-SAGHYZ): An Emergency Source of Natural Rubber, Whaley, W. G, Bowen, J. S., United States Department of Agriculture. United States Government Printing Office Washington, Misc. Publication No. 618, Jun. 1947, pages 138-141). However, this process is disadvantageous because it requires additional steps such as: removing carbohydrates in hot water, additional pebble milling and screening steps, an additional pebble mill scrubmilling step, extra drying steps, extra centrifuging, excess use of water, and the use of alkali and an acid that deteriorates the quality of rubber.
Similar processes of recovering rubber from Guayule have been proposed. For example, U.S. Pat. Nos. 2,434,412 and 2,459,369 disclose processes of recovering rubber in the form of “worms” from Guayule shrub by wet-milling with a pebble mill and separating the fibrous matter by flotation. Before wet-milling, the Guayule shrub is parboiled to remove the leaves. The rubber “worms” or whole plant are deresinated by acetone extraction and purified by dissolving in cyclohexane and filtering or centrifuging. This process was demonstrated on a pilot scale at Saltillo, Coahila, Mexico and it is known as the Saltillo process (National Academy of Sciences booklet “Guayule: An Alternative Source of Natural Rubber”). This process is disadvantageous because it requires two more extra steps, such as parboiling and deresination. Moreover, these disadvantages are in addition to the above-mentioned disadvantages of wet-milling with pebble mills.
Further, U.S. Pat. No. 2,665,317 discloses a purification process of rubber “worms” from residual plant tissue debris by scrubmilling in water in the presence of a water-insoluble soap to prevent the agglomeration of rubber “worms”. Prior to the purification step, the rubber “worms” were recovered by pebble-milling in water. This process is disadvantageous because the purified rubber “worms” contain more than 20-25% of resin, while further having all the above-mentioned disadvantages of wet-milling.
Furthermore, in U.S. Pat. No. 5,321,111, the whole Guayule shrub was treated with a guanidine salt to soften the tissue for easy hammer milling. The resulting homogenous paste was centrifuged to recover rubber. The forgoing process is disadvantageous because it uses an alkaline substance (guanidine salt) which deteriorates the rubber quality and uses extensive centrifuging to recover the rubber. Moreover, the rubber is not deresinated and contains high amounts of resin.
Accordingly, there are many disadvantages to the previously cited wet-milling processes. For instance, these processes require various unnecessary and costly extra steps such as parboiling, deresination, water washing, addition of alkali and acids, as well as several drying and purification steps. Further, the previously cited wet-milling processes use and waste an excess of water during the wet-milling steps.
Further, the foregoing rubber extraction processes are commercially costly because the extraction steps involve processing the whole plant material in solvent or water which is labor-intensive, time-consuming, expensive, and energy-intensive. Accordingly, there is a need for a process for recovering rubber from rubber-bearing plant materials that is more economical and efficient than those currently known in the art. The present subject matter addresses this need.